journal: use context instead of session for dhcp port service
journal: use context instead of session for dhcp port service

This is a preliminary patch for https://review.openstack.org/#/c/465735/

Change-Id: Ie7b15cb8237e9a9989cdfd181670cf08753b73b7

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl